Microsoft registered more than 1.5 million downloads of its free antivirus software in the week after it shipped.

The company's Security Essentials software is a basic antivirus program designed to appeal to Windows users who don't want to shell out the US$40 to $50 per year that most AV vendors charge. It was launched on Sept. 29, and by Oct. 6, the software had been downloaded more than 1.5 million times, according to a Thursday blog post by Microsoft.

The free AV software has proved popular with Microsoft's Windows 7 operating system, which is available to business users now, but set to be made widely available next Thursday. According to Microsoft, 44 percent of users are running Windows 7, followed by XP (33 percent) and then Vista (23 percent).

Though XP is not the most popular platform for Security Essentials, it's where the software is doing the most work. Microsoft counted 4 million total malware detections on more than 500,000 machines during the one-week period; 52 percent of them were on XP machines. Vista was next, with 32 percent of detections, followed by Windows 7, with 16 percent. "This follows our usual observed trend of seeing less malware on newer OSes and service packs," Microsoft said.

The U.S., China and Brazil were the top three countries reporting malware detections, with more than a quarter of all detections occurring in the U.S.

But the nature of the threats varies somewhat from country to country. "Trojans are the top detected category in the U.S., China has lots of potentially unwanted software threats, and worms (particularly Conficker) are very active in Brazil," Microsoft said. "There are also many exploits being encountered in China, which may mean these PCs do not have the latest security updates."

Security Essentials is available in 19 countries.

Antivirus vendors have predictably downplayed the effect of Microsoft's free AV efforts, but the product has received generally favorable reviews as a lightweight but generally effective security product. It competes head-on with AVG's free antivirus software, which has about 85 million users, AVG says.